This invention relates to a wide-band amplifier particularly adapted for use as an amplifier in a telephone line circuit.
In many prior art telephone exchange systems, one employed the use of electromechanical devices such as relays or stepping switches to make a connection between one subscriber line and another. In many such systems the interface between the subscriber loop or line circuit and the telephone exchange included an audio transformer by which the DC talk current to the subscriber loop is fed via two split windings on the primary of the transformer with two feed resistors in series and the AC voltage is coupled to the circuit via the secondary winding.
The ring voltage was applied via a common generator which served a group of subscriber loops or line circuits and which generator was coupled to each of the line circuits via an electromechanical relay. This approach is still widely employed in switching systems and essentially resulted in a reliable circuit based on the available technology of the time.
As one will understand, the electromechanical relay as well as the audio transformer are well known and extremely reliable components. As indicated, the prior art is replete with such circuits, and see for example a text entitled Transmission Systems for Communications by Bell Telephone Laboratories, Inc., 3rd edition, 1984.
As one can ascertain, a major disadvantage of these prior art techniques was the use of the audio transformer which as one will understand is a relatively large and bulky circuit component. Another problem, as will be explained, is the use of the common ring generator which also represented a large circuit component which required a relatively large amount of space and power consumption. As the telephone art progressed, there have been many examples of telephone exchanges or telephone systems which employ digital switching techniques and hence essentially in such switching systems, components as relays were replaced by semiconductor components. Along with these advances and the wide spread employment of semiconductor technology, there has been significant reductions in the physical size, the power requirement, and the cost and the overall performance of the line circuit. Hence line circuit as employed in a digital switching system have been implemented using integrated circuit techniques, and therefore, such circuits have resulted in substantial improvements over the line circuits employed with electromechanical devices.
For an example of such improved line circuits reference may be had to U.S. Pat. No. 4,315,106 entitled APPARATUS FOR REGULATING CURRENT SUPPLIED TO A TELEPHONE LINE SIGNAL OF THE TYPE EMPLOYED IN DIGITAL TELEPHONE SYSTEMS issued on Feb. 9, 1982 to R. C. W. Chea, Jr. and assigned to the assignee herein and incorporated herein by reference.
Reference is also made to U.S. Pat. No. 4,387,273 entitled SUBSCRIBER LINE INTERFACE CIRCUIT WITH IMPEDANCE SYNTHESIZER issued on June 7, 1983 to R. C. W. Chea, Jr. and assigned to the assignee herein and incorporated herein by reference. Such patents involve various techniques which relate to the implementation of line circuits as used in digital switching systems.
Reference is also made to U.S. Pat. No. 4,349,703 entitled PROGRAMMABLE RING SIGNAL GENERATOR issued on Sept. 14, 1982 to R. C. W. Chea, Jr. and assigned to the assignee herein and incorporated herein by reference. This patent in particular describes an improved ringing circuit which is particularly suitable for digital switching systems. In the circuit the energizing and de-energizing of the ring circuit is programmably controlled to provide for either hardware or software generation of ringing and other signal voltages. The patent also describes a digital implementation of a complete ringing circuit without the use of bulky transformers and other components. In spite of the techniques as disclosed in the above noted patents, the present solid state implementation of the line interface circuit has excluded the ringing function due to the very high voltage requirement.
In present day telephone systems, when a ringing signal is applied to a line circuit, it exhibits a voltage level of 200 volts or more in order to operate a bell ringer which is associated with many telephone subsets. This very high voltage requirement was not compatible with conventional integrated circuit techniques and hence the components employed in integrated circuits did not possess the necessary voltage requirements to handle such a large operating signal.
It is, of course, understood that the prior art being cognizant of the problem offered various circuit techniques to solve the same. For example, in a co-pending patent application entitled TELEPHONE LINE CIRCUIT AND SYSTEM filed Jan. 10, 1983 as Ser. No. 456,638 by R. C. W. Chea, Jr. et al and assigned to the assignee herein, now U.S. Pat. No. 4,456,991, there is described an implementation to the ringing problem which utilized an 80 volt bipolar semiconductor circuit which was capable of operating directly from the telephone exchange battery. U.S. Pat. No. 4,456,991 is incorporated herein by reference.
As is also well known, most telephone exchanges will supply a 50 volt DC signal to a telephone subset via the line circuit. This DC signal is furnished via a battery at the telephone exchange and is used to operate the line circuit and the subset. As one can ascertain from the above noted application, while the circuit described can operate from the telephone exchange battery, it was not able to directly handle the ringing voltage. Furthermore, the components in a line circuit must also provide protection against damage by short circuit, power cross and surge transient voltages such as those voltages produced by lightning and other types of interference. In order to protect a line circuit from damage due to such phenomena, one was forced to utilize additional components which served to increase the cost of the line circuit. Another important factor is that the line circuit should optimumly be designed so that a minimum of bias current is drawn from the exchange battery to thereby avoid a high idle high current dissipation. As one can ascertain, the number of line circuits in a telephone switching system is a function of the number of subscribers which in a large system may exceed 100,000 or more. Therefore, any reduction in power dissipation of the line circuit allows for a substantial reduction in operating costs in regard to the switching system.
It is, therefore, an object of the present invention to design a wide band, high voltage and high power amplifier which amplifier is suitable for supplying DC talk current, coupling of AC voice and high frequency metering signals and to provide a ringing voltage to the associated subscriber loops in a telephone exchange.
As will be explained, the amplifier to be described employs a flyback converter which enables superior operation. Essentially, a primary object of the present invention is to provide an operational amplifier which is suitable for interface application between the subscriber loop and the telephone exchange. Such an amplifier must possess high voltage and high power operation to enable the amplifier to implement the ringing function of the line interface circuit. The amplifier must be a wide band device having a high slew rate and a high power bandwidth factor. In conjunction with the above characteristics, the amplifier must be able to supply talk current to the subscriber loop while exhibiting low output impedance and, in general, must be a reliable and rugged device which will enable protection against short circuit, power cross and surge transients. The amplifier structure, as will be explained, can be implemented using standard integrated circuit technology where the design will eliminate the need for high voltage integrated circuit components and hence substantially reduce the cost of the line circuit while increasing the reliability and operating performance of the same.